The invention relates to methods and apparatus for rapidly, accurately setting and stabilizing gas pressure in a volume, and more particularly, to positive shut-off pressure controllers of improved accuracy, greater speed, and reduced size, complexity, and cost compared to prior art pressure controllers.
There exist a wide variety of pressure sensitive devices, such as transducers, transmitters, digital and analog gauges, pressure switches, pressure recorders, etc. having different "pneumatic" volumes associated therewith. There is a need to be able to rapidly and precisely test and calibrate such pressure sensitive devices. In order to do so, it is necessary to have a machine that can rapidly (i.e., within 10 to 15 seconds) generate selectable, precise test pressures without "overshooting".
For example, a prior art system shown in FIG. 3 shows a pressurized gas source (which can be a pressurized gas bottle), an inlet regulator 28 of conventional design supplying gas to an inlet 29 of a pressure controller which includes an inlet servo-valve 30 that feeds inlet gas into a manifold 23. Manifold 23 opens into a "test volume" 44, which includes the volume of a device being tested and calibrated. Manifold 23 also opens into an outlet servo-valve 32, which exhausts pressurized gas from manifold 23. The pressure in manifold 23 can be set by properly controlling the opening and closing of inlet valve 30 and exhaust valve 32. Generally, a minimum flow of the gas is constantly bled through regulator 28 and servo-valves 30 and 32. Pressure is controlled by operating the servo-valves to adjust flow through the volume into which pressure is being controlled.
Typically, a servomechanism 70 senses the pressure in manifold 23 and controls the action of servo-valves 30 and 32. This technique results in "dynamic" pressure control in which the pressure is constantly changing and being readjusted by action of a servo-valve. Furthermore, the accuracy and speed of operation are heavily dependent upon the magnitude of the test volume. If the test volume being "worked into" is substantially less than the maximum expected volume, the manifold volume V.sub.0 will fill up much faster, and the resulting rapid changes in P are impossible to control precisely. Also, prior art controllers tend to consume large amounts of gas due to the flow through the test volume.
Prior pressure controllers of the type shown in FIG. 3 have an inherent shortcoming in that they "mask" the presence of leaks in manifold 23, test volume 44 or any interconnecting hardware between the two and the devices being tested or calibrated.
It should be appreciated that a fundamental assumption for any pressure controller used in the testing or calibration of pressure measuring devices is that the pressure is perfectly constant and stable throughout the volume to which a reference measuring device, and the devices being calibrated are connected so that the pressure indicated by the reference device and the pressure sensed by the device being calibrated are identical. The presence of a leak between the reference device and the device being calibrated causes a pressure drop and stable but unequal pressures within the volume. For a pressure controller of the type that maintains a constant flow of gas through the manifold that will compensate for a leak, there is no way of determining the presence of a leak while the system is operating. The only way that leaks can be detected in such prior pressure controller systems is by using external industrial leak detecting substances or by shutting off the controller and monitoring the pressure in the overall volume to detect the presence of a drop or increase in pressure, indicating the presence of a leak. Another shortcoming of this type of controller is that it continuously interferes with the pressure in the volume. It therefore cannot be used in conjunction with any other controlling device such as a dead weight tester.
Variable orifice pressure regulating systems are known, in which a servo controller produces analog signals that control the orifice size of both inlet and outlet valve orifices of variable orifice valves, referred to as servo-valves. Such systems require continuous gas flow, which is undesirable because pressure is maintained under the control of the servo controller system that continually adjusts the orifice sizes of the inlet and outlet servo-valves. This results in the measured test pressure varying or "oscillating" about an average pressure. This obviously prevents the user from achieving a perfectly stable test pressure because the test pressure is always under the influence of the servo controller which is continuously adjusting around the set value. Perhaps people skilled in the art would recognize that providing high speed digital, rather than analog, control signals to open and close a valve rather than an analog signal to vary the orifice size can produce average inlet and outlet gas flows that approximate the results achievable by analog variation of the orifice sizes. However, this approach requires very high speed, high power digital inlet and outlet valves and very high speed operation of such valves. This leads to high cost, high power consumption, and probably to valve reliability problems.
The prior art does not indicate how to implement a "positive shut-off" valve system, with reasonable reliability, that provides precise, stable test pressures, avoids servo controller caused oscillations of the test pressure about an average pressure, does not mask leaks in the test pressure system, and can precisely set a pressure value without significantly overshooting the value.